When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I'm looking to fabricate (and I use this term very loosely) a rear diffuser in the style of genuine Japanese tuner / time attack vehicles to finish out my 'build.' A lot of my inspiration comes from the Top Fuel Racing FD (pictured below). I only drive HPDE, so no rules apply to my build. My father in law will be helping me with parts of the fab / install (as he ran and owned his own shop in the stainless steel industry) so I feel there's a very good possibility of this turning out half decent. I ultimately am just winging it (bad pun, ha) and any additional help would be greatly appreciated.
So we have a large sheet of 1/16th" aluminum (I believe) which I plan to have shaped to somewhere within 53-70" length (or width) and 28-32" depth. I also hope to be able to add a 20-30' angle (upward bend) to it about 7.5-13" in just before (or after) the muffler and rivet on a couple downward facing fins. So imo, I've got a rough idea of all the measurements, mounting points (and their respective distances) and relative shape to cut. Plan to use 6-8 rods, most likely 4 'front' and 4 'rear'.
Any bases I haven't covered? Any recommendations on a reputable prop rods manufacturer? How about diffuser 'overhang' (not sure if this is the appropriate term) relationship beyond the open bumper area and extended GT wing?
ps- I will be upgrading my GT wing to a larger element (1600-1800mm) that I plan to have sit higher and further back than my current GT; potentially with an interior trunk base / pedestal that'll reach out of the trunk and extend beyond / behind the vehicle (ala TFR FD style) that was apart of my original plan for when I was going to get it caged with the Voltex 7.5 etc.
My father in law also suggested adding some things I don't know the correct terminology for but he said should provide additional rigidity along weak / flex susceptible areas. He said once the alum sheet is processed, we can do another test fitting and then he can make the brackets and weld on fixed length solid prop rods (he said similar to that on the RB muffler). He's a big hotrod / ratrod guy. Has a really cool never-ending Belair hotrod that he tubbed front and rear. Been bugging him for years on end to plan a family trip to Hot August Nights in Reno, NV. He can be a bit of a curmudgeon though, ha sorry Al.
Still sound good? Anything we're missing / haven't thought of? Any constructive thoughts or suggestions are very welcome.
Heartfelt rant: I couldn't have asked for a better father in law and the dad I never had in life. Every time I get to spend time with him I feel super blessed to get to experience life with such a tremendous human being. Hopefully many years yet to come. Might even get him to come to Laguna for a ride along.
the first thing you need to do is full-stop and get that funky muffler can tucked up high as possible to the chassis before proceeding any further
.
Ain't that the truth! Haha My father in law has an issue with the can as well and has given me some friendly grief over it. Wish I could find a better solution. At one point I was supposed to get a universal Tomei Ti kit fitted and mounted up high as a center exit like the TFR but unfortunately never came into fruition.
But seriously, can't deny you are absolutely right. It's the generic RB 'race' muffler; somewhat lightweight and sounds ok for what it is. Long story short, I had to pull the plug on the track build because elements of shadiness kept coming up with the shop / shop owner and parts / tasks either weren't being completed or ordered (on time, if at all). I suppose it's my own trusting nature to give people to benefit of the doubt but over the past year or so I was able to slowly pull back and recover a significant portion (literally $20k returned) of my investment. So at least there was some silver lining and he eventually did the right thing. Needless to say I feel more than fortunate.
Any recommendations? Think someone could cut the RB can apart and optimize piping inlet / outlet relationships to reposition it? Other than my father in law, I don't know of any good welders / muffler / performance shops to ask and he only has a few light tools / machinery in his garage.
Thanks for the link! Very nice presentation of info but unfortunately it's mostly elementary theory and doesn't offer valid ways to test.
The only real world testing he suggests involves driving at illegal speeds on pubic roads and tracks / driving clubs do not allow cameras outside of the vehicle.
Update-
alum sheet has been processed and just waiting to get back together with Al for a test fitting so we can finalize the brackets plus rod lengths for final install.
I had originally planned for a custom flat bottom to be made but above mentioned shop issues, etc, so no go and here I find myself attempting to make a rear diffuser on my own. From what I've gathered in my driving experience, street cars should focus more on reducing lift vs adding downforce. Since build conception, I purposefully selected every aero piece in effort to achieve an 'optimal' front / rear aero bias.
I'm an acquaintance of the owner of Epsilon+ and at one point I asked if he'd be interested in working with me to develop a prototype s2 diffuser that could potentially be made available to the public but he wanted a substantial deposit just to start working with standard materials (not kevlar, carbon, frp, etc) and did NOT include 3D modeling. Stated that modeling is extra and at that point we're probably getting in the ballpark of $2k+ usd for a random part that really isn't going to offer much of an increase in performance (if any) and is more about playing boy racer imo. But would've been cool to have him design something what was tested. Oh well.
if you can’t find a muffler shop to handle that then it should be the last straw that convinces you to move out of that poophole state.
.
Yeah, long time ago I had an old school muffler shop I could use for odd jobs but I don't know many reputable shops anymore. I'm sure I could try digging through the internet but I try to limit my use of devices so I can focus on more productive endeavors. I can recommend a number of fine luthiers though… ha.
Mock up coming soon. Life is very busy at the moment.
ps- and yeah, CA is for people who like butt plugs.
I swear at one point I came across a half decent custom center exit Yoshimura exhaust (jdm motorcycle) on a jdm tuner type vehicle. My thought for the 8 is that I could have a competent shop weld a new '1 - 2' Y pipe attached to a new flange running into two TRS canisters serve as the replacement muffler.
Not sure if it's a feasible part / project or not but I have a feeling it'll probably be more money and effort than it's worth. Quick google took me to ebay (no surprise) and there's no shortage of affordable 'kits' out there; which would appear to make this a much easier task.
I attached the Jimny for reference and the mini was all I was able to find online.
Any constructive comments or suggestions are greatly appreciated!
And I've dropped the 8 off at Al's. He's going to see what he can think up and maybe get his buddy Jeff to help him out.
Dude seriously makes the coolest stuff. He's currently building some kind of serious dragger Volvo wagon LS swap, fully tubbed with a full cage, etc. Like the entire rear looked tube frame to me as well. His technique and execution are nothing short of incredible imo and can literally build anything. Wish I just brought my car to Jeff instead of trying to use mazda 'race' shops. There's a lot of liberty taken in how people interpret what it means to be a professional these days.
Al bailed on the DIY replacement muffler job, haha. And unfortunately RRJ didn't have a break large enough to put the bend we designed into it. So our quick fix is just to mount it with the angle set. All that's left is the rod angles and we're in business! Probably just going to spray it battleship grey or matte black.
I think I spoke too soon. Now we're entering the home stretch.
Just need to make up my mind on paint. Al suggested painting each side a different color. Sounds like a cool idea and I was going to give it a shot. I figure battle ship grey for the topside and matte black on the under.
All things considered, a few refinements to the rods and edges and hopefully next time I'll be sharing the finished part!
Snap a picture of up under/inside your trunk what you did with the 'spare tire' area. Or a link what you had done/progress pictures.
Let me start by saying that I'm far from a professional tech / engineer (or even a backyard mechanic for that matter) and have little to no experience with fabrications so you'll have to bear with any lack of terminology.
I have the basic schematics but I'm not much of a 'content creator' in terms of diligently documenting build design and progress, etc so these are the only pictures I have. I can tell you that all mounting points are composed of basic materials which you can pick up at your local hardware store and made into 'custom' mounting brackets (and I use this term very loosely, ha).
Regarding trunk area. To the best of my understanding, portions were removed via plasma cutter, etc and open areas were covered with frp or plastic panel(s). This was done with weight reduction in mind. I'm a huge proponent of the 'simplify, then add lightness' philosophy per Colin Chapman. My r3 is a street car at the end of the day with full interior, A/C, stereo, airbags, etc and my last corner balance tipped the scales at 2942 lbs (with 1/2 tank and no ballast + 'big' wheels, BBK, slew of chassis bracing, etc). I'm hoping this new setup gets me well under 2900 lbs, which would be pretty sweet imo.
Is there anything specific that you wanted to see? Let me know and I'll do my best to document it for you.
I really like the rear tail lights without the plastic cover , how did you open them ?
Thanks for noticing! I thought it was something worth trying and think it turned out half decent. Definitely much better than the oem fish bowl covers imo, yuck.
Unfortunately I do not know how this was accomplished. It was a request I made to the shop I use for service and my best guess is that it was either heat treated or cut off / away.
Slight update:
Life has been crazy and it keeps raining. Plan is to get it painted and finalized very soon. Plus take a few pics to share.
Update:
Finally found a couple potential shops to handle fab for a custom exhaust, so the current diffuser design is going to require a few additional revisions before final installation. Also, plan to adapt what I can from AJ Hartmans' diffuser design and apply it to my own. Pics will follow.
in general there seems to be a total failure to communicate what constitutes a diffuser and how it has to be configured to function in any effective manner
09-27-2023, 02:27 PM
